Soldiers started to arrive at Davis-Monthan Air Force Base over the weekend as part of President Trump’s plan to deploy up to 15,000 troops to the southern border.

The deployment is in response to groups of Central American migrants and asylum-seekers slowly making their way through Mexico.

There are about 1,200 service members deployed to Arizona, although the numbers are in flux, a spokesperson with the U.S. Northern Command said, adding that the numbers of future deployments will depend on CBP’s request for assistance and its assessment of where troops are needed.

Both Davis-Monthan Air Force Base and Fort Huachuca have been identified as locations to house the soldiers.

The Associated Press reported that more than 3,500 troops have been deployed to staging bases along the border, including about 1,000 Marines in California.

About 100 troops were seen on Friday at or near a bridge around McAllen, Texas, in the Rio Grande Valley, according to the AP. More than a dozen military members in fatigues were laying concertina wire at the northern bank of the Rio Grande, while others erected wire barriers on the bridge’s pedestrian paths.

In Arizona, pictures from the Department of Defense show soldiers and airmen assembling tents that will house the troops deployed in support of Operation Faithful Patriot.

According to the secretary of defense, the initial 7,000-troop deployment will include:

Helicopter companies to support the movement of CBP tactical personnel.

Engineer battalions to build temporary vehicle barriers and fencing.

Medical units to triage, treat and prepare for commercial transport of patients.

Temporary housing to support CBP and military personnel.

Working with light towers, barrier material, barbed and concertina wire, as well as supplying cases of ready-to-eat meals.

The soldiers are an addition to the 2,200 National Guard troops, including 580 in Arizona, already at the border to help with tasks such as vehicle maintenance, clearing vegetation and aerial surveillance.